The technology for electroless copper plating on electrical printed circuit boards is well known and well developed. It is known that proper control of the oxygen content of the plating bath is essential to the plating process. It is also known that if the oxygen content of the bath is not properly controlled, nodules of extraneous copper will be formed on the circuit board. These copper nodules can cause defects in the circuit board.
It is known that oxygen influences the plating ability of an electroless plating bath. For example, U.S. Pat. No. 4,152,467 (Alpaugh) entitled "Electroless Copper Plating Process with Dissolved Oxygen Maintained in Bath" discusses control of the oxygen content of an additive plating bath using air for oxygen control. U.S. Pat. No. 4,152,467 (Alpaugh) also contains a discussion of prior art references concerning the effect of oxygen and other gases on an electroless plating process. The discussion of the prior art references in U.S. Pat. No. 4,152,467 (Alpaugh) is hereby incorporated by reference. Also for a discussion of the effect of oxygen on a plating process see the discussion of skip plating in U.S. Pat. No. 3,900,599 (Feldstein) wherein it is suggested that skip plating can be eliminated by removing oxygen from the plating bath prior to the plating operation. It is noted that the electroless plating bath described in U.S. Pat. No. 3,900,599 (Feldstein) is a flash or strike electroless bath and subsequent electrodeposition is contemplated (see column 1, lines 65 et seq). This reference is cited here merely to show that it is known that the presence of oxygen in an electroless bath influences the plating ability of the bath.
Other references such as U.S. Pat. No. 3,666,527 (Feldstein) and U.S. Pat. No. 3,454,416 (Heymann) teach or suggest that oxygen should not deliberately or purposely be introduced in a system for electrolessly plating copper. The stabilizing effect of oxygen is described in references such U.S. Pat. No. 2,938,805 (Agens).
The use of nitrogen in an electroless plating bath is also described in the prior art. For example, U.S. Pat. No. 2,938,805 (Agens) suggests aerating a copper electroless plating bath by employing an oxygen-containing gas which is diluted with an inert gas such as nitrogen. It is however noted that the Agens patent suggests that no advantage would be obtained by using a gas containing less than about 20% oxygen in air (see column 3, lines 63-67). Other references such as U.S. Pat. No. 3,300,335 (Horvath) suggests employing a gas which can be air or an inert gas such as nitrogen in an electroless plating bath. Other references such as, for example, an article published in the IBM Technical Disclosure Bulletin Vol. 27, No. 1B, page 24 entitled "Method of Reducing the Formation of Copper Nodules in an Additive Plating Bath" by U. Schuster also suggests that air can be used to prevent the formation of nodules.
Discussions of the effect of oxygen in a nickel plating system can be found, for instance, in Chemical Abstracts, Volume 76, page 62385g and Chemical Abstracts, Volume 72, page 92799e. In particular see U.S. Pat. No. 4,550,036 (Ludwig). The use of gases such as hydrogen, helium, methane, or neon in electroless nickel plating is discussed in U.S. Pat. No. 2,819,188 (Metheny).
It is known that in continuously operating electroless plating systems, one must continuously replenish the chemicals in the plating solution and one must also continuously remove impurities from the plating solution. Systems for continuously plating generally include a plating tank and external piping. The external piping is used to remove plating solution from the tank, treat the solution, and return the treated solution to the tank. The plating solution that is removed from the tank is filtered to remove small copper particles and other impurities from the solution. As these filters remove copper particles from the solution, they become clogged and they must be replaced.
Furthermore, particles trapped on the filters act as nucleating sites for additional plating and these particles continue to grow. One result of the plating that takes place on the filters is that the filters have to be replaced at relatively short intervals. In conventional electroless plating systems, plating not only takes place on particles trapped by the filters, but plating takes place on the pipes and on all other surfaces. The plating that takes place in the external piping (including the filters) is not only wasteful, but it also harms the system and the system must be regularly stopped and the extraneous copper removed.
The extraneous plating that takes place in the external piping of prior art systems has another deleterious effect. Such plating forms small particles of copper in the plating solution. These small particles flow into the plating tank and act as nucleating sites for nodules on the circuit board that is being plated. These nodules cause defective circuit boards.